In addition to RuBP, rTCA, and rACA, a fourth pathway has been discovered for CO2 assimilation in bacteria, the 3-hydroxypropionate cycle. In this cycle, CO2 is fixed by acetyl-CoA and propionyl-CoA carboxylases ultimately forming malyl-CoA, which is then split into acetyl-CoA (to replenish the cycle) and into glyoxylate, for use in cell carbon. Past research had demonstrated this pathway only in Chloroflexus, a nonsulfur photosynthetic bacterium, but recent work has detected the pathway in several autotrophic archaea, so it seems the pathway is more widespread than previously thought.

Genomics Sheds Light on Metabolism of Cryptic Marine MicrobesTo search for genetic clues to carbon and energy metabolism in Crenarchaeota, the researchers extracted C. symbiosum DNA from its host sponge and constructed a DNA library for sequencing the symbiont’s genome. Hallam et al. then searched for representative genes linked to pathways associated with autotrophic carbon assimilation. They found many components of two pathways: the 3-hydroxypropionate cycle and the reductive tricarboxylic acid (citric acid) pathway (TCA). Both cycles involve a multistep series of chemical reactions that convert inorganic compounds—in this case, carbon dioxide—into organic carbon molecules. Though some components of the 3-hydroxypropionate cycle were missing in C. symbiosum, enough elements (including core proteins) were found to support a modified version of this pathway for carbon assimilation, using carbon dioxide.Liza Gross, Genomics Sheds Light on Metabolism of Cryptic Marine Microbes, PLoS BiologyVolume 4 Issue 4 APRIL 2006

Presence of Acetyl Coenzyme A (CoA) Carboxylase and Propionyl-CoA Carboxylase in Autotrophic Crenarchaeota and Indication for Operation of a 3-Hydroxypropionate Cycle in Autotrophic Carbon Fixation.The pathway of autotrophic CO2 fixation was studied in the phototrophic bacterium Chloroflexus aurantiacus and in the aerobic thermoacidophilic archaeon Metallosphaera sedula. In both organisms, none of the key enzymes of the reductive pentose phosphate cycle, the reductive citric acid cycle, and the reductive acetyl coenzyme A (acetyl-CoA) pathway were detectable. However, cells contained the biotin-dependent acetyl-CoA carboxylase and propionyl-CoA carboxylase as well as phosphoenolpyruvate carboxylase. The specific enzyme activities of the carboxylases were high enough to explain the autotrophic growth rate via the 3-hydroxypropionate cycle. Extracts catalyzed the CO2-, MgATP-, and NADPH-dependent conversion of acetyl-CoA to 3-hydroxypropionate via malonyl-CoA and the conversion of this intermediate to succinate via propionyl-CoA. The labelled intermediates were detected in vitro with either 14CO2 or [14C]acetyl-CoA as precursor. These reactions are part of the 3-hydroxypropionate cycle, the autotrophic pathway proposed for C. aurantiacus. The investigation was extended to the autotrophic archaea Sulfolobus metallicus and Acidianus infernus, which showed acetyl-CoA and propionyl-CoA carboxylase activities in extracts of autotrophically grown cells. Acetyl-CoA carboxylase activity is unexpected in archaea since they do not contain fatty acids in their membranes. These aerobic archaea, as well as C. aurantiacus, were screened for biotin-containing proteins by the avidin-peroxidase test. They contained large amounts of a small biotin-carrying protein, which is most likely part of the acetyl-CoA and propionyl-CoA carboxylases. Other archaea reported to use one of the other known autotrophic pathways lacked such small biotin-containing proteins. These findings suggest that the aerobic autotrophic archaea M. sedula, S. metallicus, and A. infernus use a yet-to-be-defined 3-hydroxypropionate cycle for their autotrophic growth. Acetyl-CoA carboxylase and propionyl-CoA carboxylase are proposed to be the main CO2 fixation enzymes, and phosphoenolpyruvate carboxylase may have an anaplerotic function. The results also provide further support for the occurrence of the 3-hydroxypropionate cycle in C. aurantiacus.Castor Menendez, Zsuzsa Bauer, Harald Huber, Nasser Gad'on, Karl-Otto Stetter, and Georg Fuchs, Presence of Acetyl Coenzyme A (CoA) Carboxylase and Propionyl-CoA Carboxylase in Autotrophic Crenarchaeota and Indication for Operation of a 3-Hydroxypropionate Cycle in Autotrophic Carbon Fixation. Journal of Bacteriology, February 1999, p. 1088-1098, Vol. 181, No. 4

Occurrence, biochemistry and possible biotechnological application of the 3-hydroxypropionate cycle.The 3-hydroxypropionate cycle, a pathway for autotrophic carbon dioxide fixation, is reviewed with special emphasis on the biochemistry of CO2 fixing enzymes in Acidianus brierleyi, a thermophilic and acidophilic archeon. In the 3-hydroxypropionate cycle, two enzymes, acetyl-CoA carboxylase and propionyl-CoA carboxylase, catalyze CO2 fixation. It has been shown in A. brierleyi, and subsequently in Metallosphaera sedula, that acetyl-CoA carboxylase is promiscuous, acting equally well on acetyl-CoA and propionyl-CoA. The subunit structure of the acyl-CoA carboxylase was shown to be alpha4beta4gamma4. Gene cloning revealed that the genes encoding the three subunits are adjacent to each other. accC encodes the beta-subunit (59 kDa subunit, biotin carboxylase subunit), accB encodes the gamma-subunit (20 kDa subunit, biotin carboxyl carrier protein), and pccB encodes the alpha-subunit (62 kDa subunit, carboxyltransferase subunit). Sequence analyses showed that accC and accB are co-transcribed and that pccB is transcribed separately. Potential biotechnological applications for the 3-hydroxypropionate cycle are also presented.Ishii M, Chuakrut S, Arai H, Igarashi Y. Occurrence, biochemistry and possible biotechnological application of the 3-hydroxypropionate cycle. Appl Microbiol Biotechnol. 2004 Jun;64(5):605-10. Epub 2004 Feb 28.